1. Field of the Invention
This invention relates to a hyperthermia treatment apparatus and method, and more particularly to a hyperthermia apparatus and method in which radio frequency (RF) energy is directed at a subject tissue by axially polarized dipole antennas, and a magnetic resonance imaging (MRI) machine is used to monitor the temperature of the targeted and surrounding tissues during treatment.
2. Description of Related Art
The goal of hyperthermia treatment is the destruction of tumors by raising their temperatures for an extended period of time. When certain cancerous tumors are treated by raising the temperature of the tumors to approximately 43.degree. C. for periods of 30-60 minutes via the process of hyperthermia, those tumors have been shown to be more susceptible to the effects of radiation and chemotherapy.
The treatment relies on the primitive nature of tumor vasculature. Tumor vasculature is less able than the vasculature of normal tissues to vasodilate in response to thermal stress, and thus cannot carry away heat efficiently. If tissue temperature remains elevated for an extended period of time, DNA synthesis is reduced, cell respiration is depressed, irreversible destruction of structure (and thus function) of chromosome associated proteins can occur, and autodigestion by the cells digestive mechanism can result.
Although the bioheat transfer mechanism in the body permits healthy tissue to regulate its temperature more efficiently than tumor tissue, precise control and monitoring of the treatment is still required to ensure that damage to the healthy tissue is minimized. In order for the hyperthermia process to be safe and effective, the heating during a hyperthermia procedure must be localized in the tumor volume to the maximum extent possible, and must also be uniform across that tumor volume. Particularly in cases where hyperthermia is used as an adjunct to radiation therapy and chemotherapy, significant hyperthermia to adjacent normal structure should be avoided in order to prevent hypersensitizing the healthy tissue to radiation and/or drugs. It is also essential that the heating be almost uniform within the tumor volume being treated. Relative cool spots during treatment may result in failure to kill certain cells, and perhaps a selection of cells with thermal tolerance.
The ideal power distribution for hyperthermia, therefore, is a pattern that provides broad, uniform heating over the entire treatment volume with a rather short drop-off at the tumor margins. In order to provide such a pattern, it has been proposed to use a whole body antenna array which operates at frequencies in the RF band which are capable of achieving deep penetration of the subject, and to non-invasively monitor the treatment using either a CT scanner or an MRI machine.
U.S. patent application Ser. No. 08/228,348 filed Apr. 15, 1994 (Button, Barbour, Cermignani, and Spacht), for example, discloses a hyperthermia system which combines a cylindrical phased array of axially polarized RF antennas for precise control of power distribution to the treatment area, with an MRI machine for non-invasively measuring temperatures of the targeted area during treatment. The system uses the non-invasive temperature measurement as a monitoring and feedback mechanism to increase procedural effectiveness and patient safety by allowing the operator to determine whether uniform heating is occurring in the target area, and to ensure that stray heating is not occurring in other areas.
While the system disclosed by Button et al. appears to represent a significant improvement over prior systems, it is difficult to implement in practice using conventional hyperthermia and MRI antenna arrangements. The problem is that conventional MRI machines are not designed to accommodate an entire RF antenna in addition to the RF body coil required in order to supply RF energy for the resonance process. Not only must the conventional machine be modified to accommodate the hyperthermia applicator, but the control circuitry of the conventional machine must be modified to switch between the two discrete antennas, each of which has different operating requirements and a different structure. Magnetic resonance imaging requires uniform application of very specific frequencies at relatively low energy across the area to be imaged, while hyperthermia treatment requires more focused application of energy sufficient to cause heating, which means that the RF antenna structure for the imaging portion of the treatment process is different in structure than the phased array antenna used for hyperthermia treatment. Thus, it has been assumed that dual antennas were needed in order to implement hyperthermia treatment with MRI monitoring. Because it is virtually impossible in conventional MRI machines to fit both antennas within the space provided for the single conventional MRI RF antenna, and in view of the high cost of the machines, it is unlikely that a significant number of providers will be able to make the necessary modifications or replace their existing MRI machines so as to include separate hyperthermia treatment antennas of the above-noted type, despite the advantages of hyperthermia treatment, unless some way of fitting the hyperthermia antenna into the existing machine can be found without requiring significant modifications to be made to the machine.